remove unneeded commands from ThermocyclerBackend

[rickwierenga]

the following don't have to be methods of a thermocycler backend since there are ways to retrieve this data:

• get_block_target_temperature (by querying stage/step)
• get_lid_target_temperature (by querying stage/step)
• get_total_cycle_count (just cache running protocol)
• get_total_step_count (just cache running protocol)

[rickwierenga]

get_hold_time is vague

is it current or remaining?

it should be renamed